Method and system for providing scrambled coded multiple access (scma)

ABSTRACT

A multiple access scheme is described. A first bit stream is scrambled from a first terminal according to a first scrambling signature. A second bit stream is scrambled from a second terminal according to a second scrambling signature, wherein the first bit stream and the second bit stream are encoded using a low rate code. The first scrambling signature and the second scrambling signature are assigned, respectively, to the first terminal and the second terminal to provide a multiple access scheme.

RELATED APPLICATIONS

This application is related to, and claims the benefit of the earlierfiling date under 35 U.S.C. §119(e) of, U.S. Provisional PatentApplication (Ser. No. 60/908,340) filed Mar. 27, 2007 (Attorney Docket:115426-1147), entitled “Efficient USAT Transmission Using Low-Rate TurboCodes and Scrambled Coded Multiple Access (SCMA) Techniques”; theentirety of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to communication systems, and moreparticularly to coded systems.

BACKGROUND OF THE INVENTION

Multiple access schemes are employed by modern radio systems to allowmultiple users to share a limited amount of bandwidth, while maintainingacceptable system performance. Common multiple access schemes includeFrequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA) and Code Division Multiple Access (CDMA). System performance isalso aided by error control codes. Nearly all communication systemrelies on some form of error control for managing errors that may occurdue to noise and other factors during transmission of informationthrough a communication channel. These communications systems caninclude satellite systems, fiber-optic systems, cellular systems, andradio and television broadcasting systems. Efficient error controlschemes implemented at the transmitting end of these communicationssystems have the capacity to enable the transmission of data includingaudio, video, text, etc. with very low error rates within a givensignal-to-noise ratio (SNR) environment. Powerful error control schemesalso enable a communication system to achieve target error performancerates in environments with very low SNR, such as in satellite and otherwireless systems where noise is prevalent and high levels oftransmission power are costly, if even feasible.

Thus, a broad class of powerful error control schemes that enablereliable transmission of information have emerged includingconvolutional codes, low density parity check (LDPC) codes, and turbocodes. Both LDPC codes as well as some classes of turbo codes have beensuccessfully demonstrated to approach near the theoretical bound (i.e.,Shannon limit). Although long constraint length convolutional codes canalso approach the Shannon limit, decoder design complexity preventspractical, wide spread adoption. LDPC codes and turbo codes, on theother hand, can achieve low error rates with lower complexity decoders.Consequently, these codes have garnered significant attention.

For example, conventional data transmission to and from an ultra smallterminal via satellite is usually based on Code Division Multiple Access(CDMA) technique using rate ½ or ⅓ turbo codes. CDMA spreads bandwidthto reduce the interference to adjacent satellites, whereas the turbocode provides coding gain needed to close the link. CDMA also allowsmultiple users sharing the bandwidth at the same time. However, CDMAsystems typically need a large bandwidth expansion factor to functionproperly. Additionally, CDMA systems require all signals accessing thesame spectrum at the same time to be of equal power; provision for powercontrol makes CDMA system more complicated to implement. The inherentlong propagation delay of a satellite link makes it even more difficult.

Based on the foregoing, there is a need for an access scheme that caneffectively utilize low code rates, while minimizing complexity.

SUMMARY OF THE INVENTION

These and other needs are addressed by the present invention, wherein ascrambled division multiple access (SDMA) scheme is provided.

According to one aspect of an exemplary embodiment, a method comprisesscrambling a first bit stream from a first terminal according to a firstscrambling signature. The method also comprises scrambling a second bitstream from a second terminal according to a second scramblingsignature, wherein the first bit stream and the second bit stream areencoded using a low rate code. The first scrambling signature and thesecond scrambling signature are assigned, respectively, to the firstterminal and the second terminal to provide a multiple access scheme.

According to another aspect of an exemplary embodiment, an apparatuscomprises a plurality of encoders, each encoder being configured toencode a first data stream from a first terminal and a second datastream from a second terminal using a low code rate. The apparatus alsocomprises a first scrambler configured to scramble the first bit streamfrom a first terminal according to a first scrambling signature. Theapparatus further comprises a second scrambler configured to scramblethe second bit stream from a second terminal according to a secondscrambling signature. The first scrambling signature and the secondscrambling signature are assigned, respectively, to the first terminaland the second terminal to provide a multiple access scheme.

According to another aspect of an exemplary embodiment, a methodcomprises applying joint detection and interference cancellation on areceived composite signal, wherein the composite signal includes one ormore encoded bit streams having a low code rate. The method alsocomprises estimating the encoded bit streams, descrambling the estimatedbit streams, and decoding the descrambled bit streams. The methodfurther comprises modifying the composite signal based on the decodedbit stream, and iteratively decoding bit streams of the modifiedcomposite signal.

According to yet another aspect of an exemplary embodiment, a systemcomprises a joint detector and interference canceller configured todetect and cancel interference from a received composite signal, whereinthe composite signal includes one or more encoded bit streams having alow code rate. The system also comprises a demodulator configured toestimate the encoded bit streams, a plurality of descramblers configuredto descramble the estimated bit streams, and a plurality of decodersconfigured to decode the bit streams. The system further comprises aplurality of scramblers configured to re-scramble the decoded bitstreams, wherein the joint detector and interference canceller isfurther configured to modify the composite signal for subsequentiterative decoding by the decoders.

Still other aspects, features, and advantages of the present inventionare readily apparent from the following detailed description, simply byillustrating a number of particular embodiments and implementations,including the best mode contemplated for carrying out the presentinvention. The present invention is also capable of other and differentembodiments, and its several details can be modified in various obviousrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawing and description are to be regardedas illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIGS. 1A and 1B are communications systems capable of providing ascrambled division multiple access (SDMA) scheme, according to variousexemplary embodiments;

FIG. 2 is a diagram of a transmitter configured to operate in thesystems of FIGS. 1A and 1B;

FIG. 3 is a diagram of a receiver configured to operate in the systemsof FIGS. 1A and 1B;

FIGS. 4A and 4B, are, respectively, a diagram of a system capable ofsupporting multiple transmitters using a SDMA scheme employing low rateturbo codes, and a flowchart of an associated scrambling process,according to an exemplary embodiment;

FIG. 5 is a flowchart of a process for joint detection/interferencecancellation in the system of FIG. 4A, according to an exemplaryembodiment;

FIG. 6 is a diagram of a scrambler, in accordance with various exemplaryembodiments;

FIG. 7 is a diagram of a turbo code encoder configured to useconstituent encoders, in accordance with various exemplary embodiments;

FIG. 8 is a flowchart of a process for turbo code encoding, according toan exemplary embodiment;

FIGS. 9 and 10 are diagrams of constituent encoders configured toprovide low rate turbo codes, in accordance with various exemplaryembodiments; and

FIG. 11 is a graph illustrating the performance of the SDMA system ofFIG. 4A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A method, system, and software for providing a scrambled coded multipleaccess (SCMA) scheme is described. In the following description, for thepurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the invention. It isapparent, however, that the invention may be practiced without thesespecific details or with an equivalent arrangement. In other instances,well-known structures and devices are shown in block diagram form inorder to avoid unnecessarily obscuring the invention.

Although certain embodiments of the present invention are described withrespect to low-rate turbo codes, it is contemplated that theseembodiments have applicability to low-rate codes in general (e.g., lowdensity parity check (LDPC)).

FIGS. 1A and 1B are communications systems capable of providing ascrambled division multiple access (SDMA) scheme, according to variousexemplary embodiments. A digital communications system 100 includes oneor more transmitters 101 that generate signal waveforms across acommunication channel 103 to one or more receivers 105 (of which one isshown). In this discrete communications system 100, the transmitter 101has a message source that produces a discrete set of possible messages;each of the possible messages has a corresponding signal waveform. Thesesignal waveforms are attenuated, or otherwise altered, by communicationschannel 103. To combat the noise channel 103, coding is utilized. Forexample, forward error correction (FEC) codes can be employed.

Forward error correction (FEC) is required in terrestrial and satellitesystems to provide high quality communication over a radio frequency(RF) propagation channel, which induces signal waveform and spectrumdistortions, including signal attenuation (freespace propagation loss)and multi-path induced fading. These impairments drive the design of theradio transmission and receiver equipment; exemplary design objectivesinclude selecting modulation formats, error control schemes,demodulation and decoding techniques and hardware components thattogether provide an efficient balance between system performance andimplementation complexity. Differences in propagation channelcharacteristics, such as between terrestrial and satellite communicationchannels, naturally result in significantly different system designs.Likewise, existing communication systems continue to evolve in order tosatisfy increased system requirements for new higher rate or higherfidelity communication services.

Code rate is an important factor that has a significant effect on theerror performance of the code. The choice of which code rate to operate,in turn, depends on the SNR of the environment in which the codes willbe deployed. Traditionally, low SNR environments require the use of lowcode rates (i.e., more redundancy), whereas high SNR environments canenable the utilization of higher code rates. There is a continualchallenge to devise codes that edge closer to the Shannon limit, whileminimizing complexity.

When considering turbo codes and LDPC codes, irregular LDPC codes havebeen demonstrated to achieve superior performance over turbo codes forhigh code rates, whereas turbo codes have been demonstrated to besuperior for lower code rates in low SNR environments. For very lowcodes such as ⅙ or lower, the coding industry has focused on classicalturbo code design, which can, in essence, be improved. Because turbocodes have traditionally been designed to maximize the minimum Hammingweight of systematic codewords (where the information part of thecodeword has a Hamming weight of two), it is recognized that furthercoding improvements can be made. At relatively high SNR, this approachyields good codes since two codewords are most easily confused whentheir information part is differed by two bits, owing to the recursivenature of their constituent codes.

However, for very low SNR where low code rates are traditionally used,an investigation of the erroneous turbo code frames reveals that thenumber of errors in the information part, is in general, more than two.This observation suggests that by targeting minimum Hamming weightcorresponding to information sequences with Hamming weight more thantwo, the performance of low code rates, in principle, can be enhanced.With improved design, low rate turbo codes can approach the Shannonlimit more closely, resulting in a variety of advantages forcommunication systems such as extended battery lifetime within cellularnetworks, lower transmit power within satellite communication andbroadcasting systems, etc.

FIG. 1B is a diagram of an exemplary meshed network capable ofsupporting communication among terminals with varied capabilities,according to an embodiment of the present invention.

Satellite communications system 120 includes a satellite 121 thatsupports communication among multiple satellite terminals (STs) 123, 125and a hub 127. The hub 127 may assume the role of a Network OperationsControl Center (NOCC), which controls the access of the STs 123, 125 tothe network 120 and also provides element management functions andcontrol of the address resolution and resource management functionality.The satellite 121, in an exemplary embodiment, operates as a packetswitch (e.g., at a data link layer) that provides direct unicast andmulticast communication among the STs 123, 125. The STs 123, 125 provideconnectivity to one or more hosts 129, 131, respectively. According toone embodiment of the present invention, the system 120 has a fullymeshed architecture, whereby the STs 123, 125 may directly communicate.

As previously discussed, a system in which terminals are deployed,particularly a satellite system, incompatibility problems may arise ifdifferent “generations” of terminals exist, in which one ST employsolder hardware and/or software technologies than the other.

For newer, highly capable terminals to communicate with older(typically) less capable terminals, an exchange of information regardingthe capabilities among the communicating terminals is needed.Specifically, the common air interface needs to support a discovery ofthe terminal's capabilities profile (or context information). Thesecapabilities can include encryption scheme, compression scheme,segmentation and reassembly (SAR) scheme, automatic repeat request (ARQ)scheme, Quality-of-Service (QoS) parameters, power levels, modulationand coding schemes, power control algorithms, and link adaptationcapabilities.)

Under a conventional approach, terminal profile can be readily exchangedover a network with a star topology where no peer-to-peer communicationexists. For example, in the General Packet Radio Service(GPRS)/Universal Mobile Telecommunications System (UMTS) family ofprotocols, such capabilities profiles include a packet data protocol(PDP) context and a mobility management context. In an embodiment of thepresent invention, the concepts of PDP context and mobility managementcontext are combined and the term packet data protocol (PDP) context isused in general to refer to terminal capabilities. It is recognized thatthese terminals can be mobile as well as non-mobile. In an exemplaryembodiment, this PDP context, for example, which can provide informationabout the encryption algorithm, compression algorithm, modes of datalink layer communication, and physical layer transfer capabilities iscombined by the transmit ST with the Quality of Service (QoS) of apending data flow to determine a packet transfer context to use intransmission of the flow. If a PDP context has been previouslyestablished, then the sending ST can autonomously create the packettransfer context, which both satisfies the QoS of the data flow and iscompatible with the receive ST capabilities.

According to one embodiment, the exchange of terminal profile can beexecuted over a meshed network, in a peer-to-peer manner. The STs 123,125 support the use of a negotiation procedure to determine the optimalconfiguration for transmission and reception of data. If a protocolimplements control procedures or options in newer versions (i.e.,flow-control/rate-control), older protocol versions are able to detectthe initiation as a new unsupported procedure and report the same to thepeer with minimal disruption in the flow of traffic.

The ST-ST protocol advantageously takes into account that even for peersof the same version, some capabilities may not necessarily be alwayssupported due to local temporal processing/memory/congestion-relatedconstraints. Additionally, the ST-ST protocol design provides for rapiddevelopments in data communication technology.

Incompatibility between two STs is detected by the terminal thatoriginates the traffic. Thus, potential misconfigurations or softwareincompatibilities can at least be identified, without requiringcommunication at the service level of the more capable ST. For example,one of the STs 123, 125 may need to be reconfigured in order tocommunicate with compression disabled in order to allow communicationwith an ST that does not support compression. It is noted that thecapability is not necessarily a function of solely configuration orsoftware compatibility, but may also be a function of current trafficload.

For each ST 123, 125, there exist some configuration information,including network configuration, network service provider (NSP)configuration, software configuration, and user configuration, asindicated by the NOCC 127. These configurations relate to the featuresthat the ST 123, 125 supports and offers to the user, and have a directbearing on the transmission and reception capabilities.

To facilitate the flow of data from one peer ST 123 to another ST 125 ofpossibly different generations equipped with different capabilities, apacket transfer context is employed. Such a common feature set dependson the PDP contexts of the two STs 123, 125; further, this commonfeature set may also depend on the QoS of the flow, as well as theloading and status of the two STs at that point of time. In an exemplaryembodiment, the packet transfer context is unidirectional and valid onlyfor the transmit ST to send packets to the specified receive ST; thus,the packet transfer context may be unique to a given pair of STs.

FIG. 2 is a diagram of a transmitter configured to operate in thesystems of FIGS. 1A and 1B. As seen in FIG. 2, a transmitter 200 isequipped with a channel encoder (e.g., turbo encoder) 201 that acceptsinput from an information source and outputs coded stream of higherredundancy suitable for error correction processing at the receiver (asshown in FIG. 3). The information source generates k signals from adiscrete alphabet, X. The channel encoder 201 may utilize a combinationof a constituent encoder that uses one or more constituent codes and aninterleaver 203 to implement the channel coding procedure. Turbo codesare produced by parallel concatenation of two codes (e.g., convolutionalcodes) with an interleaver in between the encoders (as seen in FIG. 4).

Essentially, the encoder 201 generates signals from alphabet Y to achannel scrambler 203, which scrambles the alphabet. That is, thechannel scrambler 203 pseudo-randomizes the code symbols. The scrambledsignals are fed to a modulator 205, which maps the encoded messages fromencoder 201 to signal waveforms that are transmitted to a transmitantenna 207.

The antenna 207 emits these waveforms over the communication channel103. Accordingly, the encoded messages are modulated and distributed toa transmit antenna 207.

The transmissions from the transmit antenna 207 propagate to a receiver,as discussed below.

FIG. 3 is a diagram of a receiver configured to operate in the systemsof FIGS. 1A and 1B. At the receiving side, a receiver 300 includes anantenna 301 that receives the waveforms emitted over the channel 103.The receiver 300 provides a demodulator 303 that performs demodulationof the received signals. After demodulation, the received signals areforwarded to a channel de-scrambler 305 to unscramble the symbols. Adecoder 307 then attempts to reconstruct the original source messages.

It is contemplated that the above transmitter 200 and receiver 300 canbe deployed in within a single wireless terminal, in which case a commonantenna system can be shared. The wireless terminal can for example beconfigured to operate within a satellite communication, a cellularsystem, wireless local area network (WLAN), etc.

FIGS. 4A and 4B, are, respectively, a diagram of a system capable ofsupporting multiple transmitters using a SDMA scheme employing low rateturbo codes, and a flowchart of an associated scrambling process,according to an exemplary embodiment. For the purposes of illustration,a communication system 400 supports multiple terminals (i.e., users)configured with respective encoders 401 a-401 n and scramblers 403 a-403n. In an exemplary embodiment, these terminals can be the transmitter200 and the receiver 300 of FIGS. 2 and 3, respectively, operating inthe satellite system 120 of FIG. 1B.

By way of example, the system 400 provides a multiple access scheme,such as SCMA, which achieves good performance with relatively lowerreceiver complexity compared to CDMA (as the number of users that sharethe same channel increases). With SCMA, each user sharing thetransmission channel is separated by user specific and scramblers 403a-403 n. Also, due to lack of spreading factor and more efficient FECcoding, a fraction of a satellite transponder is needed under the SCMAscheme, thereby lowering the operating cost.

By using low rate codes, the system 400 can achieve greater powerefficiency while spreading the spectrum, whereas conventional CDMA doesnot. Additionally, SCMA is different from another multiple accesstechnique called Interleave-Division Multiple Access (IDMA) which alsospreads with low-rate turbo-Hadamard codes but uses random interleaversas user signature. The low-rate turbo decoders in SCMA are much morestraightforward to implement since turbo-Hadamard codes require thedecoding of Hadamard codes in addition to the decoding of turbo-likecodes. Also, all the users can utilize the same scrambler hardware withdifferent initial vector (also known as “seed”), instead of differentinterleaver design. Further, using scrambling sequences as signatures issimpler than random interleaver-based signatures.

In one embodiment, each of turbo encoders 401 a-401 n utilizes the sameturbo codes. The turbo encoded sequences are then fed to the respectiveuser-specific scramblers 403 a-403 n. The scrambled sequences are thentransmitted over channel 405 to a receiver 300, which includes a jointdetector/interference canceller unit 407 that interacts with the turbodecoders 413 a-413 n to iteratively produce an estimate of the receivedcodewords. With each iteration, the turbo decoder 413 a-413 n produces abetter estimate to the joint detector/interference canceller 407 forachieving better cancellation. The information exchanged between turbodecoders 413 a-413 n and the joint detector/interference canceller 407is scrambled or descrambled via scramblers 409 a-409 n or de-scramblers411 a-411 n, respectively. Once “good” estimates of the decodedsequences are produced, they are output from the turbo decoders 413a-413 n.

Unlike conventional CDMA systems, the joint-detection/interferencecanceller 407 does not require all the signals accessing the samespectrum at the same time to be of equal power. In fact, the performanceis better when the signals are of different power level. Thus, no tightpower controls are needed. Also due to joint-detection/interferencecancellation, the system 400 provides a scheme that is much more robustagainst Rician fading, which makes it particularly more attractive forsmall mobile terminals experiencing Rician multipath fading.

Therefore, the system 400, as a SCMA system using low-rate FEC coding,requires less power to transmit data at the same speed vis-à-vis a CDMAsystem. In one embodiment, the system 400 can be operated in a randomaccess manner and does not require reservation of time slots, whichminimize the delay to one satellite round trip. Additionally, the system400, as mentioned, does not require tight power control, minimizing thecoordination needed between transmitter 200 and receiver 300. By way ofexample, potential applications will be for mobile or aeronauticalterminals. It may also have applications to enable direct broadcastsatellite (DBS) operators to provide return link over satellite via acommercial satellite using existing antenna systems.

Each user encodes its data with, for example, a rate 1/n FEC, where n isan integer larger than 3. The coded bits are then scrambled with aunique scrambling sequence and transmitted. The number of uniquesequences are virtually unlimited with common sequence generators, suchas the Gold sequences. The same generator can generate all thesequences, which are differentiated by the initial vector. It is notedthat other low rates can be utilized, m/n (e.g., less than ⅓).

In an exemplary embodiment, the scrambling sequence can be generated byselecting a pseudorandom number sequence (e.g., Gold sequence) whoseperiod is greater than the code block. On the receiver side, therespective user uses the corresponding de-scrambler and a rate 1/ndecoder to retrieve its data. The signals are modulated by the same typeof modulation, e.g., QPSK, of the same bandwidth, centered at the samefrequency and transmitted at the same time (e.g., similar to CDMA).Typically, for receivers located in a hub of a star-shaped network, theantennas can be shared.

The system 400 operates as follows. In step 421, each terminal encodesdata using the corresponding turbo encoder (e.g., 401 a-401 n). Theencoded data is then scrambled by the respective scramblers 1 . . . U(e.g., 403 a-403 n) and transmitted to the receiver 300, per steps 423and 425. Next, the received signal is processed by the jointdetector/interference canceller 407 and undergoes descrambling andre-scrambling, as in step 427. The descrambling and re-scrambling isperformed in conjunction with the decoding process, which outputsdecoded data (step 429).

FIG. 5 is a flowchart of a process for joint detection/interferencecancellation in the system of FIG. 4A, according to an exemplaryembodiment. A key enabler for this communication system 400 is thejoint-detection/interference cancellation receiver. This receiver 300includes the descramblers 409 a-409 n and the decoders 413 a-413 n aswell as all the signal estimators and interference reducers for each ofthe individual signal paths. In addition, the receiver 300 includes abuffer (not shown) to store a complete block of the composite signal.The receiver 300 employs joint detection/estimation; it is contemplatedthat any joint-detection/estimation technique may be used. In anexemplary embodiment, the receiver 300 operates iteratively to outputthe bit streams represented by the composite signal.

As seen in FIG. 5, in steps 501-507, once an entire block of compositewaveform is sampled and stored in the buffer, the receiver 300 firstuses the first descrambler (e.g., descrambler 409 a) and a turbo decoder(e.g., decoder 413 a) to estimate the first bit-stream. In step 509, thecomposite signal is modified accordingly. In this example, only one passof the turbo decoding is performed. The interference reducer thenoperates on the stored waveform given the result of the first passdecoding of the first signal. The receiver 300 then uses the seconddescrambler (e.g., descrambler 409 b) and turbo decoder (e.g., decoder413 b) to estimate the second bit-stream, and so on. When all thebit-streams have been estimated once (as determined in step 511), thereceiver 300 than returns to process the first bit-stream in a secondpass.

When all the bit-streams have been processed for the required numberpasses (steps 513-517), all the bit-streams are completely estimated anddecoded.

Alternatively, in another embodiment, all the paths can be processedin-parallel for each pass; this approach may entail more passes than theabove process.

Signal estimation, via a demodulator (not shown), plays an importantrole. In most applications of interest, this demodulator must operate atvery low signal-to-noise plus interference ratio. In one embodiment, thedemodulator is aided by two features: synchronization, and jointdetection. The initial synchronization involves use of a known pilot,which can be introduced using anyone of the techniques known in the art.For example, known pilot symbols can be introduced by multiplexing theminto the data stream, or pilot symbols may be introduced by puncturingthe encoder output. Just as each encoder 401 a-401 n employs a differentscrambling signature, each may employ a different pilot symbol pattern,thereby minimizing interference effects.

In one embodiment, the signals are transmitted in a burst mode.Accordingly, the demodulator is configured to detect the burst arrivaltimes by using, for example, a “Unique Word” pattern. It is recognizedthat any other well-known techniques may be used for this purpose. TheUnique Word patterns of the various encoders may or may not be distinct.

With respect to joint detection, this process involves iterativerefinement of the demodulation. As the iteration progresses, thedemodulation is improved through two techniques. First, as interferenceis removed, the estimation of signal parameters (e.g., frequency, symboltiming, carrier phase) is improved. Secondly, as more reliable estimatesof the data symbols become available from the turbo decoders 413 a-413n, these are used to improve the demodulator performance.

FIG. 6 is a diagram of a scrambler, in accordance with various exemplaryembodiments. In this example, a scrambler 601 receives a codeword (e.g.,“0 0 1 0 1 0”) and a scrambling sequence (e.g., “1 0 1 0 1 1”). Thescrambling sequence (or signature) can be a Gold sequence or anypseudorandom number sequence. Gold codes exhibit a number ofcharacteristics. In addition to being simple to generate, thesesequences contain roughly an equal number of zeros and ones, and areapproximately orthogonal when delayed or shifted. Also, they areorthogonal to other codes. Gold sequences can be generated usingfeedback shift registers, whose outputs are added to produce the Goldcodes. The codeword and scrambling sequence are combined by adder 603 tooutput a transmitted sequence (e.g., “1 0 0 0 0 1”).

As mentioned, use of scramblers (as opposed to interleavers) reducescomplexity. In a large system with numerous users, it is difficult todeploy a large number of interleavers that are prearranged between eachpair of sender and receiver, whereas a common scrambler with differentinitial vector (also known as “seed”) can be used for each pair ofsender and receiver. Such arrangement is substantially easier toimplement.

FIG. 7 is a diagram of a turbo code encoder configured to useconstituent encoders, in accordance with various exemplary embodiments.In this example, turbo encoder 201 employs two constituent encoders 701,703 and an interleaver 705. Although two encoders 701, 703 are describedin this scenario, the encoder 201 can provide more than two encoders toachieve various code rates. As seen in FIG. 7, the turbo encoder 201 canoptionally output the information (i.e., systematic) bit as part of theoutput code, depending on the design of the constituent encoders 701,703 and the code rates. The operation of the turbo code encoder 201 isexplained with respect to FIG. 8, as follows.

FIG. 8 is a flowchart of a process for turbo code encoding, according toan exemplary embodiment. Information bits that are to be turbo codeencoded are fed to both constituent encoders 701 and 703 (step 801). Thebits that are fed to constituent encoder 703 are, however, interleavedby interleaver 705 prior to being input to constituent encoder 703, asin steps 803-807. In steps 809 and 811, once both bit streams areencoded, the output of constituent encoders 701, 703 is punctured toachieve the desired code rate. According to an exemplary embodiment, theoutput of turbo encoder 201 can contain the unpunctured bits at theoutput of constituent encoders 701, 703 and, alternatively, theinformation bits that have not undergone any processing, as illustratedin FIG. 7.

FIGS. 9 and 10 are diagrams of constituent encoders configured toprovide low rate turbo codes, in accordance with various exemplaryembodiments. A constituent encoder 900 provides code rates of 1/14,1/12, 1/10, ⅛, ⅙, and ¼. When implemented in the turbo encoder 201 ofFIG. 7, the encoder 201 does not output the information bits given thesecode rates. For rate 1/14 turbo code, all seven parity bits of theconstituent code are transmitted. For rate 1/12 turbo code, Y₆ ispunctured from both constituent codes, for rate 1/10 turbo code Y₅ andY₆ are punctured, for rate ⅛ turbo code, Y₄, Y₅ and Y₆ are punctured,for rate ⅙ turbo code Y₃, Y₄, Y₅ and Y₆ are punctured and for rate ¼turbo code, Y₂, Y₃ and Y₄ , Y₅ and Y₆ are punctured from bothconstituent codes. The transfer function of the constituent encoder 900is given as:

${G(D)} = \left\lbrack {\frac{n_{0}(D)}{d(D)},\frac{n_{1}(D)}{d(D)},{\ldots \mspace{14mu} \frac{n_{6}(D)}{d(D)}}} \right\rbrack$where, d(D) = 1 + D + D³ n₀(D) = 1 + D² + D³ n₁(D) = 1 + D + D² + D³n₂(D) = 1 + D³ n₃(D) = 1 + D + D² n₄(D) = 1 + D n₅(D) = 1 + D²n₆(D) = 1.

As seen, the logic or circuitry for the encoder 900 encompasses adders901, 903, 905, 907, 909, 911, 913, and 915 and shift registers 917, 919and 921. Modular adder 901 receives the data input, adding it togetherwith the output of registers 917 and 921. The output of adder 901produces the parity bit Y₆ and is also fed into adders 903, 911, 909 and905. Modular adder 903 sums the signals from adder 901 and register 917,resulting in the parity bit Y₄; the summed value is additionallyprovided to adder 907. Adder 905 receives inputs from register 919 andadder 901, and produces the parity bit Y₅ at its output.

Further, adder 907 sums the values from adder 903 and register 919 togenerate parity bit Y₃. Adder 909 receives inputs from register 919 andadder 901 to produce a value, which is then fed into adder 913. Adder911 adds the value from register 921 as well as the value from adder901; the resultant value is the parity bit Y₂. As for adder 913, thisadder 913 produces parity bit Y₀ from the summation of a value fromadder 909 and register 921. Adder 915 receives inputs from adder 913 andregister 921 to generate parity bit Y₁.

FIG. 10 shows a block diagram of a constituent encoder 1000 forachieving varying code rates, such as 1/15. The transfer function of theconstituent encoder 1000 is as follows:

${G(D)} = \left\lbrack {\frac{n_{0}(D)}{d(D)},\frac{n_{1}(D)}{d(D)},{\ldots \mspace{14mu} \frac{n_{6}(D)}{d(D)}}} \right\rbrack$where, d(D) = 1 + D³ n₀(D) = 1 n₁(D) = 1 + D² n₂(D) = 1 + D² + D³n₃(D) = 1 + D + D² + D³ n₄(D) = 1 + D + D³ n₅(D) = 1 + D + D²n₆(D) = 1 + D.

To generate the parity bits according to this transfer function, theconstituent encoder 1000 has circuitry that includes modular adders1001, 1003, 1005, 1007, 1009, 1011, 1013, 1015, and 1017 and shiftregisters 1019, 1021, and 1023.

As seen, modular adder 1001 adds the output of register 1023 with thedata input and generates parity bit Y₀, which is also supplied intoadders 1005, 1003, 1011, and 1007. Adder 1003 receives input from adder1001 and register 1019, summing these inputs for a resultant value thatis fed into adder 1013. At adder 1005, signals from adder 1001 andregister 1019 are summed to yield parity bit Y₆—which is also fed intoadder 1009.

Adder 1007 adds the input from adder 1001 and the input from register1021, to provide parity bit Y₁. Parity bit Y₅ can be generated throughthe summation, at adder 1009, of the values from adder 1005 and register1021. With respect to adder 1011, inputs from adder 1001 and register1021 are added by adder 1011 and provided to adder 1015. Adder 1013receives input from adder 1003 and register 1023 to generate parity bitY₄. Parity bit Y₂ is formed by the addition of the result from adder1011 and content of register 1023. Adder 1017 receives inputs from adder1015 and register 1019 and produces parity bit Y₃ at its output. Underthis arrangement (i.e., code rate of 1/15), the turbo encoder 201 ofFIG. 7 outputs the systematic bit, by contrast to the constituentencoders 900, 1000 of FIGS. 9 and 10, respectively, in which noinformation bit is output by the turbo encoder 201.

It should be noted that, according to certain embodiments, the low rateturbo codes described herein have been developed by targeting minimumHamming weight corresponding to information sequences with Hammingweight more than two. Using this approach, a search for new turbo codeswas conducted using a uniform interleaving assumption and constrainingthe Hamming weight of input sequences to a maximum of six, for instance.

FIG. 11 is a graph illustrating the performance of the SDMA system ofFIG. 4. Specifically, in graph 1100, the performance of system 400 iscompared against Shannon's constrained capacity for QPSK modulation as afunction of the aggregated signal power per symbol time over one-sidednoise spectral density in an Additive White Gaussian Noise (AWGN)channel (e.g., using a rate 1/10 code). A relatively short 1000information bit frame length is used. As seen, up to about fivesimultaneous overlapping transmission that are close to a theoreticallybest with unlimited processing. The corresponding energy per symbol overone-sided noise spectral density ratio (Es/No) for each individual userterminal and the energy per information bit to one-sided noise spectraldensity ratio (Ebi/No) to achieve 10⁻⁵ frame error rate is shown inTable 1, as the aggregated power are evenly divided to each userterminal.

TABLE 1 No. of Users 1 2 3 4 5 6 7 Es/No (dB) per user −6.7 −6.3 −6.1−5.7 −5.0 −3.9 −2.3 Ebi/No (dB) per user 0.3 0.7 0.9 1.3 2.0 3.1 4.7

Conventional spectral spreading achieves emission reduction, but doesnot offer additional coding gain. When applied to a star-shapedsatellite network, the approach of system 400 can be applied for boththe forward link and the return link. In the forward link, the advantageis mainly the lower Ebi/No that is required by the lower rate code islower than required by the rate ⅓ or rate ¼ code previously used. Thelower operating threshold allows the terminal to accept more adjacentsatellite interference, which is often the dominating performancelimitation for small terminals. The advantage of the low-rate code canbe applied to reduced receive antenna size, or higher bit rate.

In the return link, the combination of low-rate code and SCMA allows theterminal to operate autonomously with minimum coordination with the hub.Unlike conventional CDMA, interference cancellation operates better whenthe terminals are not operating at exactly the same power, tight powercontrol is in fact not desirable. The bandwidth expansion from thelow-rate coding serves two purposes reduction of emission spectraldensity from regulator standpoint; and additional coding gain.

The processes described herein for generating low rate codes andscrambling may be implemented via software, hardware (e.g., generalprocessor, Digital Signal Processing (DSP) chip, an Application SpecificIntegrated Circuit (ASIC), Field Programmable Gate Arrays (FPGAs),etc.), firmware or a combination thereof.

In the preceding specification, various embodiments have been describedwith reference to the accompanying drawings. It will, however, beevident that various modifications and changes may be made thereto, andadditional embodiments may be implemented, without departing from thebroader scope of the invention as set forth in the claims that follow.The specification and drawings are accordingly to be regarded in anillustrative rather than restrictive sense.

What is claimed is:
 1. A method comprising: scrambling a first bitstream from a first terminal according to a first scrambling signature;and scrambling a second bit stream from a second terminal according to asecond scrambling signature, wherein the first bit stream and the secondbit stream are encoded using a low rate code, the first scramblingsignature and the second scrambling signature being assigned,respectively, to the first terminal and the second terminal to provide amultiple access scheme.
 2. A method according to claim 1, furthercomprising: generating an output data stream that includes the encodedbit streams and one or more pilot symbols for synchronization.
 3. Amethod according to claim 1, further comprising: encoding each of thebit streams to output a composite signal using turbo code encoding,wherein the encoding utilizes constituent encoding having a transferfunction:${G(D)} = \left\lbrack {\frac{n_{0}(D)}{d(D)},\frac{n_{1}(D)}{d(D)},{\ldots \mspace{14mu} \frac{n_{4}(D)}{d(D)}}} \right\rbrack$where, d(D) = 1 + D + D³ n₀(D) = 1 + D² + D³ n₁(D) = 1 + D + D² + D³n₂(D) = 1 + D³ n₃(D) = 1 + D + D² n₄(D) = 1 + D.
 4. A methodaccording to claim 1, wherein the bit streams are encoded by a pluralityof constituent encoders utilizing identical transfer functions.
 5. Amethod according to claim 1, wherein the low code rate includes rate1/14, rate 1/12, rate 1/10, rate ⅛, rate ⅙, or rate ¼.
 6. A methodaccording to claim 1, wherein the scrambling signatures include Goldsequences.
 7. An apparatus comprising: a plurality of encoders, eachencoder being configured to encode a first data stream from a firstterminal and a second data stream from a second terminal using a lowcode rate; a first scrambler configured to scramble the first encodedbit stream according to a first scrambling signature; and a secondscrambler configured to scramble the second encoded bit stream accordingto a second scrambling signature, wherein the first scrambling signatureand the second scrambling signature are assigned, respectively, to thefirst terminal and the second terminal to provide a multiple accessscheme.
 8. An apparatus according to claim 7, wherein an output datastream is generated, the output data stream includes the encoded bitstreams and one or more pilot symbols for synchronization.
 9. Anapparatus according to claim 7, wherein each encoder is configured toencode each of the bit streams to output a composite signal using turbocode encoding, wherein the encoding utilizes constituent encoding havinga transfer function:${G(D)} = \left\lbrack {\frac{n_{0}(D)}{d(D)},\frac{n_{1}(D)}{d(D)},{\ldots \mspace{14mu} \frac{n_{4}(D)}{d(D)}}} \right\rbrack$where, d(D) = 1 + D + D³ n₀(D) = 1 + D² + D³ n₁(D) = 1 + D + D² + D³n₂(D) = 1 + D³ n₃(D) = 1 + D + D² n₄(D) = 1 + D.
 10. An apparatusaccording to claim 7, wherein the encoders include a plurality ofconstituent encoders utilizing identical transfer functions.
 11. Anapparatus according to claim 7, wherein the low code rate includes rate1/14, rate 1/12, rate 1/10, rate ⅛, rate ⅙, or rate ¼.
 12. An apparatusaccording to claim 7, wherein the scrambling signatures include Goldsequences.
 13. A method comprising: applying joint detection andinterference cancellation on a received composite signal, wherein thecomposite signal includes one or more encoded bit streams having a lowcode rate; estimating the encoded bit streams; descrambling theestimated bit streams; decoding the descrambled bit streams; modifyingthe composite signal based on the decoded bit stream; and iterativelydecoding bit streams of the modified composite signal.
 14. A methodaccording to claim 13, wherein the received composite signal includesone or more pilot signals for synchronization, the method furthercomprising: performing synchronization based on the pilot signals.
 15. Amethod according to claim 13, wherein the decoding is based on a turbocode utilizing constituent encoding having a transfer function.${{G(D)} = \left\lbrack {\frac{n_{0}(D)}{d(D)},\frac{n_{1}(D)}{d(D)},{\ldots \mspace{14mu} \frac{n_{6}(D)}{d(D)}}} \right\rbrack},{where},{{d(D)} = {1 + D^{3}}}$n₀(D) = 1 n₁(D) = 1 + D² n₂(D) = 1 + D² + D³ n₃(D) = 1 + D + D² + D³n₄(D) = 1 + D + D³ n₅(D) = 1 + D + D² n₆(D) = 1 + D.
 16. A methodaccording to claim 13, wherein the low code rate includes rate 1/14,rate 1/12, rate 1/10, rate ⅛, rate ⅙, or rate ¼.
 17. A method accordingto claim 13, wherein the scrambling signatures include Gold sequences.18. A system comprising: a joint detector and interference cancellerconfigured to detect and cancel interference from a received compositesignal, wherein the composite signal includes one or more encoded bitstreams having a low code rate; a demodulator configured to estimate theencoded bit streams; a plurality of descramblers configured todescramble the estimated bit streams; a plurality of decoders configuredto decode the bit streams; and a plurality of scramblers configured tore-scramble the decoded bit streams, wherein the joint detector andinterference canceller is further configured to modify the compositesignal for subsequent iterative decoding by the decoders.
 19. A systemaccording to claim 18, wherein the received composite signal includesone or more pilot signals for synchronization.
 20. A system according toclaim 18, wherein the decoding by the decoders is based on a turbo codeutilizing constituent encoding having a transfer function,${{G(D)} = \left\lbrack {\frac{n_{0}(D)}{d(D)},\frac{n_{1}(D)}{d(D)},{\ldots \mspace{14mu} \frac{n_{6}(D)}{d(D)}}} \right\rbrack},{where},{{d(D)} = {1 + D^{3}}}$n₀(D) = 1 n₁(D) = 1 + D² n₂(D) = 1 + D² + D³ n₃(D) = 1 + D + D² + D³n₄(D) = 1 + D + D³ n₅(D) = 1 + D + D² n₆(D) = 1 + D.
 21. A systemaccording to claim 18, wherein the low code rate includes rate 1/14,rate 1/12, rate 1/10, rate ⅛, rate ⅙, or rate ¼.
 22. A system accordingto claim 18, wherein the scrambling signatures include Gold sequences.23. A system according to claim 18, wherein the composite signalrepresents a plurality of bit streams corresponding to a plurality ofsatellite terminals.